add docs/rootless.md

AkihiroSuda · AkihiroSuda · commit ce6d4c9a9ff0 · 2018-11-06T17:39:30.000+09:00
Signed-off-by: Akihiro Suda &lt;suda.akihiro@lab.ntt.co.jp&gt;
diff --git a/docs/rootless.md b/docs/rootless.md
@@ -0,0 +1,41 @@
+# Running containerd as a non-root user
+
+A non-root user can execute containerd by using [`user_namespaces(7)`](http://man7.org/linux/man-pages/man7/user_namespaces.7.html).
+
+For example [RootlessKit](https://github.com/rootless-containers/rootlesskit) can be used for setting up a user namespace (along with mount namespace and optionally network namespace). Please refer to RootlessKit documentation for further information.
+
+## Daemon
+
+```console
+$ rootlesskit --net=slirp4netns --copy-up=/etc \
+  --state-dir=/run/user/1001/rootlesskit-containerd \
+  containerd -c config.toml
+```
+
+* `--net=slirp4netns --copy-up=/etc` is only required when you want to unshare network namespaces
+* Depending on the containerd plugin configuration, you may also need to add more `--copy-up` options, e.g. `--copy-up=/run`, which mounts a writable tmpfs on `/run`, with symbolic links to the files under the `/run` on the parent namespace.
+* `--state-dir` is set to a random directory under `/tmp` if unset. RootlessKit writes the PID to a file named `child_pid` under this directory.
+* You need to provide `config.toml` with your own path configuration. e.g.
+```toml
+root = "/home/penguin/.local/share/containerd"
+state = "/run/user/1001/containerd"
+
+[grpc]
+  address = "/run/user/1001/containerd/containerd.sock"
+
+[plugins]
+  [plugins.linux]
+    runtime_root = "/run/user/1001/containerd/runc"
+```
+
+## Client
+
+A client program such as `ctr` also needs to be executed inside the daemon namespaces.
+```console
+$ nsenter -U --preserve-credentials -m -n -t $(cat /run/user/1001/rootlesskit-containerd/child_pid)
+$ export CONTAINERD_SNAPSHOTTER=native
+$ ctr -a /run/user/1001/containerd/containerd.sock pull docker.io/library/ubuntu:latest
+$ ctr -a /run/user/1001/containerd/containerd.sock run -t --rm --fifo-dir /tmp/foo-fifo --cgroup "" docker.io/library/ubuntu:latest foo
+```
+
+* `overlayfs` snapshotter does not work inside user namespaces, except on Ubuntu kernel
